cylinder #2 dead

[germanlake]

need lots of help! my 1987 yamaha 70hp2 stroke lost #2 cylinder compression.
pulled cover off side of engine to view piston sides and the rings.
oh oh! aluminum smeared on side of piston rings and groves in piston side.
pulled head and top of piston has no holes.
cant afford to replace so hope it is found to be repairable after inspection by machinist. we plan on doing all disassembly and reassembly if found to be repairable.
how can i tell the cause and fix same before lighting the fire again.
any and all help and advice will be greatly appreciated!
thanks,
chris

[99yam40]

carb problem is my thoughts
Ran lean on air/ fuel

[germanlake]

thanks so much! would there be any way for me to know for sure to avoid a repeat?

[TownsendsFJR1300]

How does the spark plug look compared to the others?

If it's white(ish) that cylinder is running lean(likely a clogged carb).

Running Yamaha's RingFree helps keep the carbs clean and the top end about eliminating that (if that is indeed your issue).

Not using a fuel stabilizer in the engine(even if you run it dry) STILL leaves a little fuel in the float bowl that will varnish up and can clog your jets..


Over sized pistons and rings (for boring the block) are available for your engine too(part#'s 8 and 9);

http://www.boats.net/parts/search/Ya...TON/parts.html

[germanlake]

thank you everyone, the plugs all looked identical, first thing we checked.
we put stabil in all our gas but if we are successful we will use the ringfree from the dealer.
is it ok to only bore the bad cylinder for OS piston and just hone and re-ring the other 2?

[TownsendsFJR1300]

IMO, I would punch them all out and pretty much have a new top end across the board.

The engines already fully broke down, your talking two more bores, and two pistons (your already buying rings).

You want (I believe its) no more than 10% difference compression across the board.

With all new parts, you should be much closer than that once all assembled..

[germanlake]

all 3 it is! thanks so much for the help! i am sure once we start reassembly i will have many questions.so comforting to know there are such helpful people.

any know a good source for service manual? a friend has a Clymer or some such name of a manual that includes my motor but sure not as good as Yamaha. it is so nice that the tightening torque is embosed on the parts, very impressive

[TownsendsFJR1300]

The Clymers manual is probably more user/backyardian friendly than the Yamaha manual.

The Yamaha manuals generally assumes you've been working on outboard engines for awhile..

Having both probably wouldn't be a bad idea.

[robert graham]

After you get the motor repaired and carbs cleaned consider the installation of a fuel/water separator type canister filter on your transom to help prevent water/dirt/debris from entering your carbs and plugging jets....Walmart sells an Attwood brand unit for $28 and it's an easy item to install....very good preventive maintenance item!....good luck with your mechanical work!

[germanlake]

thanks much for the separator tip! i will google it to see what i need to pipe it up, i have 2 tanks on the back of my pontoon that i just switch the supply hose back and forth as they empty. had just filled them both with non ox. fuel with stabil so was fresh.

[ausnoelm]

toss the Clymer manual in the bin if doing a rebuild, it will be useless
make sure you do a proper carb rebuild, and take particular care to clean all parts, and keep then clean during assembly

[germanlake]

thanks for the tips on the manual and the neatness. i will remind my sons who are doing the work. they have rebuilt numerous snowmobiles but never and outboard so somewhat familiar with the basics, but a totally different animal.
thanks again!

[amahaork]

when I done my, first i found good machine shop in my area and talk to them.
then get home and bared block, put all in usps small boxes, per cylinder, per unit, etc. and marked them. made block clean.
then I meticulously eliminate all aluminium embedded into scorned cylinder walls, using acid.
I had 12 broken bolts in aluminium block. had to dig them and fix threads. it took me awhile, every hole was challenge.
then back to shop and talk about bores and pistons and rings, types and sizes. then two days later back from shop with bored block and wash it aggressively with detergent.
then assembly with many new and some used parts, and all new gaskets.
and new fuel pumps.
first season used pre-mix to addition to precise blend.
now second season with ringfree